The present invention relates to filtering of power supplies. More particularly, the present invention relates to limiting the amount of in-rush current that occurs upon applying voltage to the power supply filter.
Power supplies are filtered to prevent noise from being transferred from the power supply to the load. Noise may emanate from various sources, including ripple from an alternating current (AC) power source that is rectified to provide direct current (DC) voltage from the power supply. Furthermore, loads that receive energy from the power supplies may emanate noise that returns to the power supply and is potentially distributed to additional loads being powered.
To prevent noise from being distributed by a power supply to its associated loads, a power supply filter is used to suppress the noise. The power supply filter generally contains capacitive and/or inductive components that obstruct frequencies that contribute to the noise. To provide a steady level of DC voltage, large capacitors may be connected in parallel with the power supply to suppress any voltage ripple or other noise. When the power supply is first energized, the large capacitor may contain little or no charge and as a result a large in-rush current may occur to instantaneously charge the capacitor.
For safety, power supplies employ over-current protection such as a fuse or circuit breaker. The large in-rush current that may occur may be much greater than the normal operating current being drawn by the load of the power supply. Therefore, the large in-rush current blows the fuse or trips the circuit breaker, and proper operation of the power supply is inhibited.
Attempts to prevent the in-rush current from aggravating the over-current protection have included operators manually connecting a resistor in series between the charge capacitor and power supply for a period of time and then removing the resistor to directly reconnect the capacitor across the power supply output. For relatively high-voltage power supplies, the charged capacitor presents a dangerous potential for electrical shock when the operator is manually handling leads from the capacitor.
Thus, power supply filters are a necessary feature of power supplies, but they result in additional problems and hazards due to the in-rush current problem.
Embodiments of the present invention provide systems and methods that address the shortcomings of power supply filters resulting from the in-rush current problem. The embodiments provide various circuit elements for limiting the amount of in-rush current that exists upon energizing the power supply. The various circuit elements also allow the filtering to be unrestricted even though the in-rush current is limited to avoid tripping any over-current protection that may be provided for the power supply. The various embodiments include power supply filters positioned within power distribution panels.
One embodiment involves placing a resistor in series with the filtering capacitor. A relay is in parallel with the resistor and is left in an open circuit condition upon energizing the power supply so that current passes through the resistor when charging the capacitor. Once the filtering capacitor is at least partially charged, the relay is switched to a closed circuit condition to short circuit the resistor, and thereby directly connect the filtering capacitor across the voltage nodes of the power supply that the load may be connected across. The relay may be controlled in various ways. In one embodiment, the relay is controlled based on a comparison of the voltage across the filtering capacitor to the power supply voltage. In another embodiment, the relay is controlled based on expiration of a set amount of time.
Another embodiment involves placing a first filtering capacitor across first and second input nodes, a second filter capacitor across first and second output nodes, and a common mode choke between the input and output nodes. The common mode choke is disposed such that current flows between the first input node and first output node by passing in one direction through the common mode choke and current flows between the second output node and the second input node by passing through the common mode choke in a second direction opposite the first. The placement and operation of the common mode choke allows the first and second capacitors to have a relatively small capacitance.